In the prior art, a method has been known for controlling the welding position and condition or the like of the arc welding by picking up the image of a portion to be welded such as a molten metal, a welding tip, a wire or a welding groove with the use of industrial TV camera (ITV) and by analyzing the image picked up.
However, the arc flame has a more intense brightness than that of the surroundings (e.g., the molten metal) and contains much infrared and ultraviolet rays so that difficult problems are encountered in detecting the welding portion.
Therefore, the following solutions have been worked out in the prior art.
One solution is to devise an image pickup or an optical filter. In Japanese Patent Laid-Open No. 51-72938, for example, the radiation from the portion being welded is transmitted only in the visible and near infrared ranges by providing filters for limiting infrared rays, heat rays, colors and quantities of light, respectively, and by using a silicon vidicon image pickup tube in the TV camera. In Japanese Patent Laid-Open No. 59-212172, on the other hand, the image is picked up by an infrared TV camera through an infrared filter. In Japanese Patent Laid-Open No. 58-159980, moreover, the filter is dispensed with by using a CCD (i.e., a charge-coupled device or solid image sensor) as the image pickup device.
Another method for solving the influences of the arc is to provide a high-speed shutter in the industrial TV camera. In Japanese Patent Laid-Open No. 58-7776, for example, the shutter is opened only upon short-circuiting of the arc by catching the chance of arc disappearance. In Japanese Patent Laid-Open No. 59-202178, on the other hand, the shutter is opened in the pulse welding operation only when the arc attenuates at the timing of a base current.
Still another method for picking up the image of a portion being welded while discriminating the arc is to provide a light source for the image pickup. In Japanese Patent Laid-Open No. 55-42185, for example, the light source has a spectral distribution different from that of a disturbance light such as the arc, and the wavelength to be detected falls within a wavelength range in which the quantity of the disturbance light is smaller than that of the light source. In Japanese Patent Laid-Open No. 59-191574, on the other hand, a light source is provided for projecting a beam forward of the running direction of a welding torch to represent the image of the welding groove in a contour defined by shadows.
What can be commonly said among the various prior art examples described above is to separate and remove the image to be picked up and the arc partly because the object to be picked up is the portion to be welded such as the molten metal and partly because the arc is the disturbance light obstructing the image pickup.
Since, however, the arc has a high intensity and a wavelength covering substantially the whole range, it can hardly be completely removed, and the system therefor has to become complex and expensive.
In the case of narrow gap welding operation, on the other hand, some example monitors the arc itself. In Japanese Patent Laid-Open No. 55-45554, for example, the luminance distribution of the arc is determined to locate the center of rotations of the welding core wire in the gap. In Japanese Patent Laid-Open No. 58-187268, on the other hand, the movement of the torch is controlled by providing an optical detector at the leading end of the torch and by comparing the outputs of the detector.
However, the above examples are limited to the narrow gap welding operations but cannot be applied to the lap, fillet and butt joint welding operations the present invention aims at.
A first object of the present invention is to solve the various problems thus far described and to detect the groove position of an arc welding by observing the arc itself without adding any special device.
Since, on the other hand, the wire is let off from the welding torch obliquely with respect to the normal plane, its leading end will fail to follow the welding line so that precision welding cannot be expected, unless the height of the welding torch is properly positioned. Hence, the control of the height of the welding torch is an important factor for precision welding.
Thus, a second object of the present invention is to accomplish high precision welding by detecting whether or not the height of the welding torch is proper, by the use of a binary image obtained through direct observation of the arc.
In the prior art, as has been described hereinbefore, efforts have been made to eliminate the influences of the arc by all means from the image picked up. However, the arc has a high intensity and a wavelength covering substantially the whole range so that the influences of the arc can hardly be completely eliminated. If possible, however, it is seriously difficult to take the image by using the industrial TV camera commerically available. If, moreover, the welding end point or the corner point is to be detected by analyzing the taken image such as the molten metal, this detection has to be accompanied by complicated analyses. Of these, it is necessary to discriminate whether welding port has reached the end point or the corner point or has gone out of position.
On the other hand, the sensor using an external light source such as a laser beam is almost compatible with a method of detecting the step of the welding groove with linear information (e.g., a slit beam). This makes it impossible to stably detect the welding end point or the corner point. Thus, the method of the laser beam or wire contact type cannot detect errors caused by the thermal strain of the welding operation.
A third object of the present invention is to detect the welding end point and the internal and external corner points of the lap and fillet joint welding operations by detecting the arc in the visible range with an existing industrial TV camera, requiring neither any external light source such as a laser nor any special device for separating and removing the arc, and by analyzing the pattern of the arc.
Contrary to the aforementioned prior art method of detecting the welding position, the method of detecting the welding groove gap is exemplified by detecting changes in the step of the work by the use of the laser beam. Despite this fact, however, there exists no method of detecting the changes in the height of the step of the work from the image picked up during the welding operation by the industrial TV camera. On the other hand, there are a number of problems with the method of detecting the gap through the direct image pickup because the arc is far more intense in luminance than the surrounding light.
There is another method of burying the gap detected. However, mere changes in the welding conditions (e.g., the current, voltage and rate) would make it difficult to set the welding conditions, because of the problem of burning through the work, and would reduce the allowance for the gap. In still another method, the gap is buried by the weaving process using an automatic welder. This method cannot deal with the real-time changes in the size of the gap due to the thermal strain during the welding operation.
A fourth embodiment of the present invention is to solve the various problems described above and to accomplish the adaptability control of the groove gap by directly observing the arc itself without any special device to recognize the pattern thereby to detect the welding groove gap.
In the automatic welding operation using a robot, the detection of the work contact of the welding wire is necessary for detecting the welding start point.
In one method of detecting the contact of the wire with the work, this contact is detected in terms of electric conduction.
Specifically, a power supply circuit is prepared between the welding torch and the work, and the wrist of the robot is moved in horizontal and vertical directions, as shown in FIG. 31. The power supply circuit is established when a conductive wire 5 projecting from a welding torch 4 comes into contact with the wall 7a or 7b of a work 7. The point intersecting the wall, i.e., a welding start point A is detected by storing the horizontal and vertical coordinates Px and Py in the robot.
However, this method of detecting the wire contact in terms of the electric conduction may fail to electrically detect the merely physical contact of the wire and the work, because this contact is electrically insulated if the wire or work is covered with an oxide film such as rust. In order to eliminate the influences of the insulating film, therefore, a high power source is required in the circuit so as to break down the insulation.
A further method is to detect the welding start point by processing the image with the CCD camera. Since the information of the CCD camera is two-dimensional, the method is defective in that the accuracy in the vertical direction is insufficient although the accuracy in the horizontal direction is acceptable.
A fifth object of the present invention is to detect the contact of the wire with the work in terms of image information.
Now, generally speaking, the control point of the welding robot falls at the tip of the electrode wire which is fed from the leading end of the welding torch carried by the wrist. The locus is so controlled and corrected that the desired locus may be followed by the tip of the electrode wire having a desired length.
It is necessary for starting the welding operation to detect the position of the wire of the welding robot at present. This detection of the welding start point is accomplished by either a method of detecting the displacement of the work by holding the welding wire in touch with the work or a method of monitoring the distance between the robot and the work with the laser beam.
However, the former method of having the wire touching the work is influenced by the state of the tip of the welding wire so that it finds it difficult to detect the displacement of the work within an accuracy of 1 mm or less.
On the other hand, the latter method using the laser beam has to be accomplished by an expensive system which is difficult to set. Another difficulty of this system is the use as a robot sensor in the field.
A sixth embodiment of the present invention is to detect the welding start point with high accuracy by neither changing the relative positions of the camera, the light source and the welding torch nor requiring any wiring inching and to construct the system at a reasonable cost.